Gas chromatography is commonly practiced at temperatures above ambient temperature with the separation column temperature usually controlled by installing it in a convection oven. The oven thermal control systems usually rely on ambient for downward temperature adjustment of the column assembly of a gas chromatography instrument. The maximum rate of this downward temperature adjustment is typically achieved by turning off any heating and applying forced air convection where possible. Because the rate of heat exchange is proportional to the temperature difference, this rate of cooling slows as the oven approaches ambient temperature. The more massive and more insulated the oven is, the slower this process can be. To cool more quickly than the physical limits imposed by convection and conduction for a specific design, or to cool to temperatures below the ambient limit, a different means to remove heat must be used, such as cryogenic liquids or a thermoelectric cooler.
Cryogenic liquids are liquids having low boiling temperatures at ambient pressure. Cryogenic liquids can be delivered mechanically and released at the point of the desired cooling. The rapid vaporization and expansion of the resulting gas results in a large drop in temperature from evaporative cooling and Joule-Thomson cooling. Most commonly, cylinders of liquid carbon dioxide are valved and discharged into the entire interior oven of a gas chromatography instrument to provide Joule-Thomson cooling. This is an inefficient and expensive process, because entire cylinders can be discharged quickly to cool the ovens of the gas chromatography instrument repeatedly. In thermoelectric cooling (or Peltier cooling), a solid-state active heat pump transfers heat from one side of the device to the other. Thermoelectric cooling, however, is practically limited to smaller sizes of gas chromatography instruments and provides limited rates of heat transfer. Thermoelectric cooling also requires large electrical currents with significant heat dissipation for its heat pumping process. Because cryogenic liquids can cool effectively, as compared to thermoelectric cooling, discharge of a cryogenic liquid into the oven of a gas chromatography instrument is the conventional method for cooling a gas chromatography (GC) column.
The development of low thermal mass (LTM) gas chromatography column assemblies eliminated the effective mass of the conventional gas chromatography oven as an obstacle to rapid heating and cooling of the gas chromatography column. However, the cooling rate of such a conventional LTM column assembly is still dependent upon the convection conditions, the difference between the column temperature and ambient temperature, and the low thermal mass device surface area. While cooling faster than ordinary gas chromatography ovens because of the smaller mass, the time spent cooling the column is still unproductive waiting time and should be minimized in order to maximize the productive utilization of a gas chromatography instrument. Also, low thermal mass gas chromatography instruments are unable to lower the column temperature below the ambient temperature.
Thus, what is needed are devices and methods for quickly cooling the column of a gas chromatography instrument and maintaining subambient temperatures, without the wasteful and expensive use of relatively large amounts of cryogenic liquids.